1. Field of the Invention
The present invention is a communication system, more particularly it is a secure communications system for maintaining secure communication between subscribers.
2. Prior Art
Prior art secure communication systems which utilize at least one working variable for enciphering and deciphering secure messages transmitted therein, do not remotely select these working variables for purposes of retransmission of a secure message between subscribers in the system. These prior art systems utilize a working variable which must be known to all subscribers receiving the secure message. This working variable, known by the subscribers, must be inserted into their enciphering/deciphering means in order to maintain secure communication. If each subscriber to the system has a different working variable, the one initiating the message in such a system must have at his disposal the working variable of the subscriber he wishes to call so that he may insert it in his enciphering/deciphering means in order to maintain a secure message between subscribers. This requires a substantial inventory of working variables at the place of message initiation, and reception, thus minimizing the security of the system.
Another feature of prior art secure communication systems, which has limited desirability from a security viewpoint, is the requirement that in order to change the working variables utilized in these systems these variables must be changed in accordance with a predetermined schedule, known to all subscribers in the system; thus, there is once again a minimization of security.
In the secure communication system of the present invention, the security liabilities of prior art systems are overcome by providing for an automatic reiterative replacement for the working variables of the system subscribers, and by providing a means by which the working variable of the subscriber which is called is remotely selected for purposes of retransmission by the subscriber initiating the call. By reiteratively replacing the working variables automatically, there is no need for conforming to a rigid schedule known to all parties. By accomplishing remote selection and reiterative replacement by some means external to the subscribers to the system, at some central location, an absolute maximization of system security is provided. This is due to the singular remote location of the necessary information, as opposed to the multiplicity of locations, one at each subscriber, necessary in prior art systems, as well as the fact that the actual working variable which is utilized, at any given time, is unknown to all subscribers in the system, the setting of the enciphering/deciphering means of the subscribers being accomplished automatically with information received from a remote selection means. Furthermore, the security of the system of the present invention is enhanced due to the ease of reiterative replacement, which may occur as often as once per message instead of once per day, or once per plurality of messages, as in prior art systems.
Prior art subscription television systems employing remote selection of switch setting information in order to allow the subscriber to receive a scrambled subscription television picture cannot provide for remote selection of a working variable in the sense that the switch setting information received is not utilized to transmit a secure message between the subscriber and another subscriber, but rather merely to receive information already existent.